While community-based sports clubs can provide children with opportunities for physical exercise, they often need a high degree of support from the children’s parents. The present study investigated the motivations for mothers to support these clubs based on previous studies on motivation for volunteering. We conducted an online survey of Japanese mothers whose children in grades three to six participated in community-based junior sports clubs (N = 800). The results indicated that high levels of harassment among parents, as well as volunteer requirements, increased the mothers’ feelings of being burdened, causing them to be less likely to continue supporting the club. We also found that the satisfaction with volunteer participation had positive effects on the mothers’ intentions to continue to support the clubs and reduced feelings of being burdened. Furthermore, justice sensitivity moderated the relationship between the sense of burden and the intention to continue supporting the clubs. The results of the present study are expected to be examined with respect to other volunteer activities.